1. Field of the Invention
The present invention relates to an optical element holder and a light scanning unit used in an image forming apparatus, such as a printer, a facsimile machine, and a copying machine.
2. Description of the Related Art
Recently, a digital printer, a copying machine, and the like are provided with a laser scanner unit (hereinafter, abbreviated to LSU) that performs scanning using a laser beam in order to form an image on the surface of an image carrier, such as a photoconductive drum.
The LSU has optical components, such as a laser light source, a polygon mirror, an fθ lens, and a reflection mirror. These optical components are disposed fixedly at specific positions on the frame of the LSU. In this LSU, light emitted from the laser light source is deflected on the surface of the polygon mirror to be scanned equiangularly, and then scanned by the fθ lens at a constant velocity on the image carrier used as an image forming surface. The fθ lens is a lens elongated in the light scanning direction, and it is directly placed on the housing of the LSU (for example, see JP-UM-A-6-43617).
Meanwhile, there has been a need for a color image forming apparatus in recent years to increase a speed, and a tandem-type image forming apparatus is now being adopted, in which image forming units for black, yellow, magenta, and cyan are disposed separately.
For the tandem-type image forming apparatus to achieve a reduction in size, there has been proposed a method of scanning light on the image carrier in each image forming unit using a single polygon mirror (for example, see JP-A-2005-288825).
FIG. 10 is a schematic cross section of the LSU disposed at the lower position of the tandem-type image forming apparatus in the related art. As is shown in FIG. 10, a photoconductive drum 100a for black, a photoconductive drum 100b for cyan, a photoconductive drum 100c for magenta, and a photoconductive drum 100d for yellow are provided.
Laser beams emitted from four laser light sources (not shown) corresponding to four dyes are scanned equiangularly by a single polygon mirror 101 and scanned on the respective photoconductive drums at a constant velocity by a group of fθ lenses. The group of fθ lenses is formed of scanning lenses 103 and correction lenses 102a, 102b, 102c, and 102d for the respective laser beams.
The scanning lenses 103 corresponds to the fθ lens disclosed in JP-UM-A-6-43617 and chiefly plays a role of scanning a laser beam at a constant velocity. The correction lenses 102a, 102b, 102c, and 102d chiefly play a role of correcting bows in the sub-scanning direction (a phenomenon that the scanning line of a light beam deflected by the deflector is curved).
Such correction lenses 102a, 102b, 102c, and 102d are unnecessary in the image forming apparatus in the related art disclosed in JP-UM-A-6-43617. However, for the image forming apparatus of FIG. 10, it is necessary to form images of laser beams on the respective photoconductive drums 100a through 100d, and because correction by the scanning lenses 103 alone is insufficient, there arises a need to provide the correction lenses 102a through 102d for the respective photoconductive drums.
As is shown in FIG. 10, in order to prevent a correction lens for a given dye from interfering with the optical path of the laser for another dye, the correction lenses 102a, 102b, 102c, and 102d cannot be placed directly on the bottom surface 104 of the LSU.
Hence, the correction lenses 102a, 102b, 102c, and 102d have to be disposed spaced apart from the bottom surface 104 to form a space for allowing a laser beam to pass through. The correction lenses 102a, 102b, 102c, and 102d therefore need to be fixed to the housing of the LSU at the both ends (the both end portions in the horizontal direction deemed as the longitudinal direction of the lenses).
The LSU, however, is provided with heat sources, such as the laser light source and a motor to rotate the polygon mirror 101, and due to heat from these heat sources, the correction lenses 102a, 102b, 102c, and 102d undergo thermal expansion. In the event of thermal expansion when the correction lenses 102a, 102b, 102c, and 102d are fixed at the both ends, bending occurs in the vertical direction with respect to the longitudinal direction (light scanning direction) . When bending occurs in the correction lenses 102a, 102b, 102c, and 102d as described above, displacement occurs in the light scanning direction, which results in a color shift in a tandem-type color copying machine or the like.